It is known to carry out the machining of mechanical pieces, more specifically in numerical control grinding machines, with in-process checking, i.e. by utilizing gauging heads or other checking and/or measuring apparatuses that detect dimensions of the piece during the machining and signal the reaching of a preset dimension in order to control the stop of the machining and/or the transition to a subsequent machining phase. European patent application No. EP-A-0791873 discloses a specific method of machining in a grinding machine, according to which the machined piece is a crankpin of a crankshaft that rotates with orbital motion about the main axis of the shaft while the grinding-wheel slide translates with respect to such axis.
In the machining of this type of pieces, the reaching of the extremely high standards of accuracy that are nowadays required is difficult owing to the concurrent requirement of ever shorter production times. In fact, the machining cycles must be very short, with a high ratio between the machining feed rate of the grinding-wheel in movement towards the pin (that determines the stock removal rate), and the shaft rotation speed. As a consequence, the cross-section of the machined pin has, instant-by-instant and up to a final phase, for example the “sparkout” phase, an accentuated axial asymmetry, that is radial dimensions that considerably vary—in relation with the requested accuracy —from one point of the circumference to the other. This makes the in-process checking, carried out by known heads with one or more feelers that contact just certain pin surface areas, necessarily separate from the area that is actually machined at that instant, particularly difficult. The method of machining according to the mentioned patent application No. EP-A-0791873 foresees a sequence of steps including a stop at an intermediate position during the grinding-wheel slide machining feed towards the pin (while the slide continues to displace in order to follow the orbitally rotating pin without removing material) when the in-process checking head signals the exceeding of a preset diameter dimension, by a certain margin higher than the desired final dimension. The method also foresees that the dimensions detected in one or more pin revolutions, when the grinding-wheel slide is in the intermediate position, be processed for calculating an average value from which there is determined the amount of the subsequent grinding-wheel slide feed necessary for reaching the desired dimension. The checking head is retracted and inactive in the last part of the machining, that foresees the grinding-wheel slide machining feed of the determined amount and a final “sparkout” phase, whereupon the grinding-wheel slide is stopped again.
The method according to the mentioned European patent application attempts to solve the problems introduced by the contrasting rising requirements in terms of production speed and accuracy of the results.
However, the proposed machining cycle is burdened by controls and additional operations, for interrupting and then continuing the machining feed of the grinding-wheel slide before reaching the desired dimension, that are in contrast with the machining time reduction requirements. Furthermore, such additional operations contribute to deteriorate the involved electric and mechanic components and represent possible additional causes of grinding machine failures and malfunctions.
Apparatuses for the in-process checking of crankpins in orbital motion are described in the international patent application No. WO-A-9712724, filed by the same applicant of the present patent application. Such apparatuses include checking heads with a Vee-shaped reference element that rests on two points on the surface of the pin to be checked, and a movable feeler that contacts such surface between the two rest points and transmits the displacements to transducer means along a direction that substantially corresponds to the bisecting line of the “Vee”.
The apparatuses produced by the owner of the present patent application according to the teachings of the patent application No. WO-A-9712724 guarantee excellent results from a metrological point of view, utmost applicative simplicity and low forces of inertia, and the standards of performance that are reached in the related applications in grinding machines confirm the remarkable quality and reliability of the adopted solutions.
However, the requirements for higher machining speed while maintaining unchanged or increased the degree of accuracy, cause the previously mentioned inconveniences even with such apparatuses.